REGIONAL TILT OF THE MOUNT-STUART BATHOLITH, WASHINGTON, DETERMINED USING ALUMINUM-IN-HORNBLENDE BAROMETRY - IMPLICATIONS FOR NORTHWARD TRANSLATION OF BAJA BRITISH-COLUMBIA

We have developed a new quantitative method to estimate paleohorizonta l in; granitic plutons using the aluminum-in-hornblende (AH) barometer , The method is used to correct previously published paleomagnetic dat a from the 93-96 Ma Mount Stuart batholith of the Cascades Mountains, Washington State, for the effects of postemplacement tilting, AN barom etry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage require d for pressure estimation, High-contrast back-scattered electron imagi ng was used to ensure that the analyzed hornblendes were not significa ntly affected by subsolidus alteration. The success of the AH barometr y is indicated by two observations, First, increases in the Al content of the hornblendes are governed almost entirely by a pressure-sensiti ve tschermak-type substitution, Second, amphibole-plagioclase thermome try indicates that assemblage equilibration occurred at or very near m agmatic conditions (approximate to 650 degrees C) and that temperature has a negligible effect on our pressure estimates, AH barometry resul ts indicate that the depth of crystallization across the batholith dec reases systematically from approximate to 0.3 GPa in the northwest to approximate to 0.15 GPa in the southeast, consistent with independent barometry for the contact aureole of the batholith and regional struct ural and stratigraphic relations, Using a best-fit planar-tilt model a nd bootstrap analysis of uncertainties, we estimate that the paleohori zontal plane has a strike of 43 degrees +/- 30.4 degrees and dip of 7 degrees a 2.0 degrees southeast (+/-95% confidence). Our estimated pal eohorizontal allows us to restore the paleomagnetic data of Beck et al , (1981) and to estimate the original paleolatitude of the Mount Stuar t batholith, Beck et al. found that the southern part of the batholith yielded a number of sites with a well-defined high-coercivity remanen ce. The carrier of this remanence was not resolved, but the following four lines of evidence strongly suggest that the published directions were acquired shortly after emplacement of the batholith, (I) The ''st able'' sites all came from the shallowest and most rapidly cooled port ions of the batholith as indicated by our AH results and concordant K/ Ar ages for hornblende/biotite pairs, (2) The high coercivity componen t was always normal in polarity, which is consistent with emplacement of the Mount Stuart batholith at the end of the Cretaceous long normal , (3) Sites from the batholith and the contact aureole gave similar di rections. (3) The directions show no indication of tilt-related smeari ng, After restoration, the paleomagnetic data indicate 42 degrees +/-1 1 degrees clockwise rotation and 3100 +/- 600 km of northward offset ( +/-95% confidence), This result verifies Beck et al.'s original interp retation that the Mount Stuart batholith originated at the paleolatitu de of northern Mexico.